Julia has a datatype called Set, that is mathematical equivalent of a Set. From the angle of Julia you can think Set as an Array which contains just unique values and cannot be ordered or sorted.

So let’s dive in, we can create a set as shown below:

set = Set([1, 2, 3, 4])

Output:

Set{Int64} with 4 elements:
  4
  2
  3
  1

So we see that we use an array [1, 2, 3, 4], this is been passed to a Function called Set() like this Set([1, 2, 3, 4]) and out we get out a set which is stored in a variable called set. Why don’t you try out Set([1, 2, 3, 4, 1, 2, 3, 4]) and see what happens?

We will also create another set as shown:

another_set = Set((3, 4, 5, 6, 7))

Output:

Set{Int64} with 5 elements:
  7
  4
  3
  5
  6

Now we will try to push in a new value 8 in another set. Note we use push!() function which means the argument passed to it gets modified:

push!(another_set, 8)

Output:

Set{Int64} with 6 elements:
  7
  4
  3
  5
  8
  6

In the above piece of code we give another_set as argument to the push!() and we add 8 to it, so another_set get modified.

Let’s now try to push 8 again to another_set:

push!(another_set, 8)

Output:

Set{Int64} with 6 elements:
  7
  4
  3
  5
  8
  6

As you can see from the above example, a set contains just unique values and if you push a value that already exists, it will not grow.

Unions

Julia has a union() function with which you could get a set’s union as shown:

union(set, another_set)

Output:

Set{Int64} with 8 elements:
  7
  4
  2
  3
  5
  8
  6
  1

Intersection

Julia has a intersect() function with which you can find intersection of two sets as shown:

intersect(set, another_set)

Output:

Set{Int64} with 2 elements:
  4
  3

Difference

Julia has setdiff() function with which you can find the difference of two sets as shown, in the belowcode it means get all the elements of set that are not there in another_set:

setdiff(set, another_set)

Output:

Set{Int64} with 2 elements:
  2
  1

Other Operations

You can find number of elements in a set using length() function as shown:

length(set)

Output:

To get the type of set you can use the typeof() operator as shown:

typeof(set)

Output:

Set{Int64}

Set is just another collection so you can use for loop as shown:

for element in set
    print(element, ", ")
end

Output:

4, 2, 3, 1,

Rather than using the cumbersome for, you can print the values of set as shown:

print(join(set, ", "))

Output:

4, 2, 3, 1

In the above example join(), joins the set elements with a string ", " and outputs this string: "4, 2, 3, 1", this when passed to println() outputs 4, 2, 3, 1.

You can’t sort a Set

You can’t sort a set, unlike Array and Tuple a set is unordered:

sort(set)

Output:

MethodError: no method matching sort(::Set{Int64})
Closest candidates are:
  sort(!Matched::AbstractUnitRange) at range.jl:1014
  sort(!Matched::AbstractRange) at range.jl:1017
  sort(!Matched::SparseArrays.SparseVector{Tv,Ti}; kws...) where {Tv, Ti} at /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.5/SparseArrays/src/sparsevector.jl:1912
  ...

Stacktrace:

 [1] top-level scope at In[12]:1

 [2] include_string(::Function, ::Module, ::String, ::String) at ./loading.jl:1091

 [3] execute_code(::String, ::String) at /home/karthikeyan/.julia/packages/IJulia/a1SNk/src/execute_request.jl:27

 [4] execute_request(::ZMQ.Socket, ::IJulia.Msg) at /home/karthikeyan/.julia/packages/IJulia/a1SNk/src/execute_request.jl:86

 [5] #invokelatest#1 at ./essentials.jl:710 [inlined]

 [6] invokelatest at ./essentials.jl:709 [inlined]

 [7] eventloop(::ZMQ.Socket) at /home/karthikeyan/.julia/packages/IJulia/a1SNk/src/eventloop.jl:8

 [8] (::IJulia.var"#15#18")() at ./task.jl:356

Converting Set to Array

You can’t convert a set to array as easy as passing a set to an Array() as shown:

Array(set)

Output:

MethodError: no method matching Array(::Set{Int64})
Closest candidates are:
  Array(!Matched::LinearAlgebra.SymTridiagonal) at /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.5/LinearAlgebra/src/tridiag.jl:142
  Array(!Matched::LinearAlgebra.Tridiagonal) at /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.5/LinearAlgebra/src/tridiag.jl:583
  Array(!Matched::LinearAlgebra.AbstractTriangular) at /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.5/LinearAlgebra/src/triangular.jl:157
  ...

Stacktrace:

 [1] top-level scope at In[13]:1

 [2] include_string(::Function, ::Module, ::String, ::String) at ./loading.jl:1091

 [3] execute_code(::String, ::String) at /home/karthikeyan/.julia/packages/IJulia/a1SNk/src/execute_request.jl:27

 [4] execute_request(::ZMQ.Socket, ::IJulia.Msg) at /home/karthikeyan/.julia/packages/IJulia/a1SNk/src/execute_request.jl:86

 [5] #invokelatest#1 at ./essentials.jl:710 [inlined]

 [6] invokelatest at ./essentials.jl:709 [inlined]

 [7] eventloop(::ZMQ.Socket) at /home/karthikeyan/.julia/packages/IJulia/a1SNk/src/eventloop.jl:8

 [8] (::IJulia.var"#15#18")() at ./task.jl:356

You need to use list comprehension trickery as shown:

array_from_set = [element for element in set]

Output:

4-element Array{Int64,1}:
 4
 2
 3
 1

As though we do not believe we can make Array from Set we check the typeof of array_from_set

typeof(array_from_set)

Output:

Array{Int64,1}

Coverting Set to Tuple

We do not get an Array from set using the Array() thing, but we can get a Tuple, using Tuple() as shown:

Tuple(set)

Output:

(4, 2, 3, 1)

That’s sad, because Array is treated as unwanted second class citizens, but may be there could be a reason for that which I am unaware.

Pop out a element from a Set

We can pop out a element from a set using the pop!() function as shown:

pop!(set)

Output:

Note that since we have used bang !, it modifies the passed variable set and the popped out value 4 get removed from set as shown below:

set

Output:

Set{Int64} with 3 elements:
  2
  3
  1

size() returns a Tuple and its for multidimensional data structure like Array, so it won’t work on Set:

size(set)

Output:

MethodError: no method matching size(::Set{Int64})
Closest candidates are:
  size(!Matched::BitArray{1}) at bitarray.jl:104
  size(!Matched::BitArray{1}, !Matched::Integer) at bitarray.jl:107
  size(!Matched::Core.Compiler.StmtRange) at show.jl:1874
  ...

Stacktrace:

 [1] top-level scope at In[19]:1

 [2] include_string(::Function, ::Module, ::String, ::String) at ./loading.jl:1091

 [3] execute_code(::String, ::String) at /home/karthikeyan/.julia/packages/IJulia/a1SNk/src/execute_request.jl:27

 [4] execute_request(::ZMQ.Socket, ::IJulia.Msg) at /home/karthikeyan/.julia/packages/IJulia/a1SNk/src/execute_request.jl:86

 [5] #invokelatest#1 at ./essentials.jl:710 [inlined]

 [6] invokelatest at ./essentials.jl:709 [inlined]

 [7] eventloop(::ZMQ.Socket) at /home/karthikeyan/.julia/packages/IJulia/a1SNk/src/eventloop.jl:8

 [8] (::IJulia.var"#15#18")() at ./task.jl:356

Get the Notebook

Well that’s it for sets, get the Jpyter notebook for it here https://gitlab.com/data-science-with-julia/code/-/blob/master/sets.ipynb